Alkaline surfactant polymer (ASP) floods in sandstone reservoirs are associated with silicate scaling of production wells for oil (petroleum). Silicate scaling has been a significant problem in ASP-flooded fields in China and Canada.
ASP flooding is a tertiary enhanced oil recovery (EOR) method designed to lower interfacial tension (IFT), water wet the formation, and decrease water mobility necessary to produce residual oil. The ASP flood uses a combination of alkali, surfactant and polymer to achieve these results. The flood requires extensive surface equipment used for mixing the components and injection through strategically placed wells.
ASP floods often induce silicate scaling as the alkaline phase increases the pH to a level that some naturally occurring silicon species dissolve. Common types of silica deposit encountered is the precipitation of magnesium silicate and calcium silicate. This deposition is strongly dependent on the pH and the temperature of the system as well as the actual concentration of magnesium and/or calcium itself. In systems above pH 9, magnesium silicate is very likely to form due to the presence of magnesium hydroxide and silicate ions. Other hydroxide salts such as calcium, strontium and sodium can react with silicate ions; however, the resulting products are much more soluble and will revert back to the ionic phase more readily. Additionally, if calcium is present in the connate water then contact in the production well with high pH ASP water will promote the formation of calcium carbonate scale (calcite). Calcium, similarly to magnesium, can serve as a nuclei or bridge to aid in the formation of colloidal silicate particles and increase silicate scale.
Silicate scale is very difficult to remove from oil wells once it has formed and traditionally has required mechanical removal. Various polymers and other chemicals have been developed that act as inhibitors of amorphous silica and magnesium silicate scales.
In summary, the type and amount of silicate scale observed is dependent on several factors including pH, magnesium concentration, and the magnesium to calcium ratios. Over the lifetime of an ASP flood these factors will change across the field and individual wells giving varying scale precipitations with time and from well to well.
It would be desirable to develop new and/or alternative compositions and methods that would inhibit or prevent the formation of silicate scales.